A solid oxide fuel cell (SOFC) is a fuel cell using an oxide ion conductor as an electrolyte. The SOFC, as advantages thereof, (1) has high power generation efficiency, (2) can deal with a variety of fuels, (3) has broad adaptability ranging from a small-sized dispersed power source to a large scale system substituting thermal power plants, and (4) does not need a Pt catalyst.
However, lowering of an operation temperature (current operation temperature: 750° C.) is a key to wide use of the SOFC. Lowering of the operation temperature (for example, to 600° C.) enables (1) improvement of cell durability (chemical stability), (2) use of inexpensive casing (made of inexpensive stainless steel), and (3) shortening of start-stop time.
For attaining the purpose, various proposals have been made so far.
For example, Patent Literature 1 discloses a method of manufacturing a solid oxide electrolyte film including:
(a) depositing an electrolyte layer including yttria-stabilized zirconia (YSZ) on the surface of a silicon wafer coated with silicon nitride by using DC or RF magnetron sputtering;
(b) depositing a first electrode layer on the upper surface of the electrolyte layer;
(c) partially removing the silicon wafer by etching to expose a silicon nitride layer;
(d) removing the exposed silicon nitride layer thereby exposing the lower surface of the electrolyte layer; and
(e) forming a second electrode layer on the lower surface of the electrolyte layer.
Patent Literature 1 describes that
(A) a thin nano-scale electrolyte film is obtained by the method described above, and
(B) the SOFC can be operated at a low temperature by reducing the thickness of the electrolyte film.
For lowering the operation temperature of the SOFC, it is necessary to use a solid oxide having high oxygen ion conductivity even at a low temperature for the electrolyte film, or decrease the thickness of the electrolyte film. When an MEMS (Micro Electro Mechanical System) technique is used as described in Patent Literature 1, it is possible to manufacture an SOFC in which a thin film electrolyte including YSZ is formed on the surface of an Si substrate, that is, an SOFC having a diaphragm structure.
However, since the YSZ is an isotropic ion conductor, no improvement can be expected in the ion conductivity except for decreasing the thickness. Further, since the thickness of the electrolyte film is at a submicron order (10 to 200 nm), the electrolyte film tends to be cracked when the layer is formed to a diaphragm structure. Further, stress concentration caused by the difference of the linear expansion coefficient occurs in a support portion due to external disturbance such as upward and downward temperature fluctuation, tending to break the electrolyte diaphragm. Accordingly, it has been difficult to increase the diameter of the electrolyte diaphragm.
Further, the method described in Patent Literature 1 requires a deposition step of silicon nitride as a passivation film on a silicon substrate in view of the process. The silicon nitride film deposition step makes the manufacturing step longer to result in an increase of the manufacturing cost.